The Immune System and Antibody Diversity
Antibodies (immunoglobulins) are produced by the B-cells (B-lymphocytes) of the immune system of animals for the purpose of recognizing and contributing to the elimination of foreign substances found within the host mammal. Any foreign substance, typically but not exclusively a protein, that induces such an antibody response by the host, is termed an antigen. Upon antigen stimulation, mature B-cells differentiate into plasma cells that proliferate and secrete antigen specific antibodies into the serum.
Immunoglobulins are Y-shaped, tetrameric molecules consisting of two relatively long polypeptide chains called heavy (H) chains and two shorter polypeptide chains called light (L) chains. Each pair of arms of the Y-shaped structure has specific antigen binding properties and each arm is referred to as an antigen-binding fragment (Fab region). The tail (or base) of the Y structure is a crystallizable fragment (Fc) that includes the binding site for activating cytolytic activity (the Fc region).
Immunoglobulin molecules possess variable regions that are responsible for their specific antigen recognition. The features that distinguish one immunoglobulin variable region from another are collectively termed the antibody "idiotype," which is derived from the Greek for "private form." In the next step in classification and nomenclature, the variable region idiotypes contain and are defined by a plurality of determinants, termed "idiotopes." These idiotopes consist of three dimensional configurations of various peptides that make up the polypeptide chains of the Fab regions; like foreign protein antigens, each idiotope is immunogenic and capable of eliciting anti-idiotype immune responses (antibodies specific for an individual idiotope or group of idiotopes). (See, e.g., Kunkel, H. G., et al., Science 140:1218 (1963); Oudin, J., and Michel, M., Cr. Acad. Sci. (Paris) 257:805 (1963).)
The variable region is encoded for by V.sub.H, D and J.sub.H gene segments for the heavy chain and V.sub.L and J.sub.L gene segments for the light chain. (See Tonegawa, S., Nature 302:575 (1983). It is the combination of these genetic elements that creates distinct antigenic determinants, or idiotopes, within the immunoglobulin variable regions. Idiotopes may be shared (e.g., "public") or not shared (e.g., "private"). These terms and the concept of shared idiotopes are explained below.
An antibody whose formation is stimulated by administration of an antigen can bind the antigen through non-covalent bonds. This binding is postulated to be based on topographic complementarity of the antibody binding site with the binding site of the antigen. (the inventor does not, however, presume any specific means through which such binding may actually occur.) The binding site of the antigen, which is thereby recognized by the antibody, is termed the "epitope" and the binding site on the antibody is termed the "paratope." (Jerne, N., Ann. Immunol. (Inst. Pasteur) 125C:373 (1974).) A paratope may serve as an idiotope, i.e., the paratope may stimulate an anti-idiotypic response in which, like the original epitope, the anti-idiotypic antibodies bind to the paratope. If an anti-paratope anti-idiotope antibody structurally mimics the antigen it is called the "internal image" of the antigen. In addition to these anti-paratopic anti-idiotypes that represent the configuration of the original antigen, other anti-idiotypic antibodies define antibody and T-cell receptor idiotopes that participate in the regulation of immune responses. These idiotypes are termed "regulatory idiotypes" and they are not necessarily the internal images of the original antigen. (See, e.g., Burdette, S. and Schwartz, R., New Eng. J. of Med. 317:219 (1987).) The utility of internal image anti-idiotypes in immunotherapy has been shown in experimental systems. Internal image anti-idiotypes have been used, for example, as surrogates for antigens in generating specific immune responses against viral, bacterial, and parasitic infections, and cancers. (See, e.g., Herlyn, D., et al., Science 232:100 (1986); Raychaudhuri, S., et al., J. Immunol. 137:1743 (1986)).
There exists a large number of antibodies that will bind to a particular antigen. There are a few reasons for this diversity. Firstly, there are hundreds of different V.sub.H and V.sub.L gene segments. Different gene segments can be expressed in different combinations for different antigenic binding properties. Secondly, V.sub.H or V.sub.L genes may be combined with different D and J gene segments. Thirdly, different V.sub.H and V.sub.L chains may be combined in different ways. These factors are the cause of the diversity of immunoglobulin expression observed in mammals. Somatic mutations occur in B-cells that further increase diversity.
Even at the epitope level of specificity, the body may produce more than one antibody molecule reactive with that epitope. These antibodies may differ in the constant region or the variable region of the light or heavy chain. Within a species, differences may be seen in the heavy chain constant region. These differences are known as "isotypes" and refer to different immunoglobulin classes and subclasses within the immune system, e.g., IgG1, IgG2, IgG3, IgG4, IgM, IgA1, IgA2, and IgE in humans. These structural differences provide different immunoglobulin molecules with specific effector functions such as ability to fix complement, to increase phagocytosis by macrophages, etc. Within a particular individual's antibody isotype, additional inherited differences in heavy chain constant region structure may occur and are termed "allotypes." The allotypic regions of the heavy chain constant region are derived from genes that are inherited from each of the individual's parents. The differences in antibody molecule seen in the immunoglobulin variable regions are a result of the use of particular V.sub.H and V.sub.L genes by the B-cell as it differentiates into an antibody producing plasma cell. Thus, an antibody reactive with a particular epitope present on an antigen may use different heavy chain constant regions, different light chain constant regions (either kappa or lambda), or may select different V.sub.L or V.sub.H gene segments. Different combinations of these variable region gene segments may be constructed that maintain the appropriate complementarity and reactivity with the epitope. Anti-idiotypic antibodies react with particular sections (idiotopes) of the immunoglobulin variable regions. Antibodies reactive with defined epitopes are more likely to utilize particular V.sub.H or V.sub.L gene segments than randomly selected antibodies. The particular antigenic determinant or "epitope" within the variable region that is defined by an anti-idiotype antibody is known as an idiotope. Thus, many idiotopes exist within immunoglobulin variable light and variable heavy chain segments, or are defined by three dimensionally contiguous regions of heavy and light chains.
Because of the tremendous diversity generated within the immunoglobulin gene system, cross-reactivity of anti-idiotypes rarely is seen and not expected. In cases where different antibody molecules are reactive with the same antigenic determinant (epitope), cross-reactive idiotypes are more frequently observed. This is based on the need to select appropriate V.sub.L and V.sub.H gene segments that would have a complementary structure to the antigenic determinant (epitope). Thus, the finding of shared anti-idiotypes suggests that these antibody molecules may be reactive with similar epitopes (in the immunoglobulin case, idiotopes). However, similar V.sub.L and V.sub.H gene segments may be used for reactivity to other antigenic determinants. In these cases, anti-idiotypes may cross-react with antibodies that do not have similar antigen binding specificities. Thus, the discovery by the inventor that lymphomas have shared idiotopes suggests that either these idiotopes are reactive with similar antigenic determinants (for example, antigens associated with pathogenesis) or that the malignant B-cells are pre-disposed to select particular V.sub.H or V.sub.L gene segments in the malignant process.
Antibodies are only one component of the animal immune response system. Whenever antibodies bind to foreign protein, they effectuate a wide range of events that may eventually lead to the destruction of the recognized antigen. Antibodies are manufactured by B-cells, which are a type of lymphocyte. A B-cell carries a sample of the particular antibody which that cell manufactures on its cell surface. As previously discussed, stimulation of this cell surface antibody by binding of an antigen will stimulate the B-cell to differentiate into a plasma cell that secretes serum antibodies in a positive feedback loop. For example, when an antigen is introduced into the bloodstream, it may come into contact with existing circulating serum antibodies. In such a case, the antigen would not reach the particular B-cell and would not cause such stimulation and differentiation. However, if the antigen introduced is in excess of the available circulating antibody, then stimulation of further antibody production can occur through this mechanism.